Nitric oxide (NO) is an endogenously generated, lipophilic signaling molecule that has been implicated in the maintenance of vascular homeostasis, modulation of oxygen radical reactions, inflammatory cell function, post-translational protein modification and regulation of gene expression. In addition, nitric oxide-derived species display separate and unique pharmacological properties, specifically can mediate oxidation and nitration of biomolecules such as, for example, unsaturated fatty acids.
Various reactions yield products capable of concerted oxidation, nitrosation and nitration of target molecules. For example, nitric oxide may react with superoxide (O2−) to yield peroxynitrite (ONOO−) and its conjugate acid, peroxynitritrous acid (ONOOH), the latter of which may undergo homolytic scission to form nitrogen dioxide (.NO2) and hydroxyl radical (.OH). In some instances, biological conditions may favor the reaction of ONOO− with CO2 which yields nitrosoperoxycarbonate (ONOOCO2−), which rapidly yields .NO2 and carbonate (CO3−) radicals via homolysis or rearrangement to NO3− and CO2. During inflammation, neutrophil myeloperoxidase and heme proteins such as myoglobin and cytochrome c catalyze H2O2-dependent oxidation of nitrite (NO2−) to .NO2 resulting in biomolecule oxidation and nitration that is influenced by the spatial distribution of catalytic heme proteins. The reaction of .NO with O2 can also produce products that can be substrates or reactants for nitrosation and nitration. For example, the small molecular radius, uncharged nature and lipophilicity of .NO and O2 facilitate concentration of these species in biological membranes in a process referred to as the “molecular lens” effect. The increase in concentration induced by .NO and O2 solvation in hydrophobic cell compartments accelerates the normally slow reaction of .NO with O2 to yield N2O3 and N2O4. Finally, environmental sources also yield .NO2 as a product of photochemical air pollution and tobacco smoke.
Nitration of fatty acids by .NO2 can occur through several methods. For example, during both basal cell signaling and tissue inflammatory conditions, .NO2 can react with membrane and lipoprotein lipids. In both in vivo and in vitro systems, .NO2 has been shown to initiate radical chain auto-oxidation of polyunsaturated fatty acids via hydrogen abstraction from the bis-allylic carbon to form nitrous acid and a resonance-stabilized bis-allylic radical. Depending on the radical environment, the lipid radical species can react with molecular oxygen to form a peroxyl radical, which can react further to form lipid hydroperoxides then oxidized lipids. During inflammation or ischemia, when O2 levels are lower, lipid radicals can react to an even greater extent with .NO2 to generate multiple nitration products including singly nitrated, nitrohydroxy- and dinitro-fatty acid adducts. These products can be generated via hydrogen abstraction, direct addition of .NO2 across the double bond, or both, and in some cases, such reactions may be followed by further reactions of the intermediate products that are formed. Hydrogen abstraction causes a rearrangement of the double bonds to form a conjugated diene; however, the addition of .NO2 maintains a methylene-interrupted diene configuration to yield singly nitrated polyunsaturated fatty acids.
The reaction of polyunsaturated fatty acids with acidified nitrite (HNO2) can generate a complex mixture of products similar to those formed by direct reaction with .NO2, including the formation of singly nitrated products that maintain the bis-allylic bond arrangement. The acidification of NO2− can create a labile species, HNO2, which is in equilibrium with secondary products, including N2O3, .NO and .NO2, all of which can participate in nitration reactions. The relevance of this pathway as a mechanism of fatty acid nitration is exemplified by physiological and pathological conditions wherein NO2− is exposed to low pH (e.g., <pH 4.0). This may conceivably occur in the gastric compartment, following endosomal or phagolysosomal acidification or in tissues following-post ischemic reperfusion.
Nitrated linoleic acid (LNO2) has been shown to display robust cell signaling activities that are generally anti-inflammatory in nature. Synthetic LNO2 can inhibit human platelet function via cAMP-dependent mechanisms and inhibits neutrophil O2− generation, calcium influx, elastase release, CD11b expression and degranulation via non-cAMP, non-cGMP-dependent mechanisms. LNO2 may also induce vessel relaxation in part via cGMP-dependent mechanisms. In aggregate, these data, derived from a synthetic fatty acid infer that nitro derivatives of fatty acids (NO2-FA) represent a novel class of lipid-derived signaling mediators. To date, a gap in the clinical detection and structural characterization of nitrated fatty acids has limited defining NO2-FA derivatives as biologically-relevant lipid signaling mediators that converge .NO and oxygenated lipid signaling pathways.